From afar, Earth looked much as it does today when the Neogene period began. But looks are deceiving. Mountains rose, and sea levels fell. The climate cooled and dried. Species were forced to adapt or die.

Though close to where they are today, the continents began the Neogene by crashing into each other. India continued its slow-moving collision with Asia, which had already started the giant push-up of the Himalaya that continues today. Italy pushed into Europe, giving rise to the Alps. Spain butted France, and the Pyrenees rose. Faulting, stretching, thinning, and lifting created parts of the Rocky, Sierra Nevada, and Cascade Mountains in North America. The high mountains altered air circulation and weather patterns, contributing to the drier and cooler climate.

The Arctic ice cap grew and thickened. Snow and ice fell on the high mountains, locking up water far from the oceans. Sea levels plummeted, exposing land bridges between Africa and Eurasia and between Eurasia and North America. Eventually, South America moved north and merged with North America, forming the Isthmus of Panama.

Species Spread Out

The continental connections gave animals that had evolved in isolation access to new lands. Elephants and apes wandered from Africa to Eurasia. Rabbits, pigs, saber-toothed cats, and rhinos went to Africa. Elephants and rhinos continued across the Bering Strait to North America. Horses went the other way. Ground sloths migrated from South America to North America; raccoons scurried south. Even rodents may have hopped Pacific islands en route to Australia from Southeast Asia.

As the climate changed, many of the great forests that carpeted the continents from shore to shore and from Pole to Pole slowly gave way to grasslands, a habitat more suited to the cooler and drier weather. But that hardiness came with less nutrition. Plant-eating animals had to adapt in order to survive. Horses evolved stronger, enamel-protected teeth and flourished. So too did ruminants such as bison, camels, sheep, and giraffes, whose compartmentalized stomachs are well adapted to digesting grass. Many of the grazers were quick and roamed in herds—new tricks for survival out in the open. Their predators were also forced to adapt.

In the oceans, a new type of large brown algae, called kelp, latched onto rocks and corals in cool shallow waters, establishing a new habitat favored by sea otters and dugongs, a marine mammal related to the elephant. Sharks grew and dominated the seas once again. Megalodon, the biggest shark of all, was nearly 50 feet (15 meters) long.

Meanwhile on land, Asian and African apes diverged and then, several million years later, hominins split from their closest African ape ancestors, the chimpanzees. Adapted to two-footed walking, early hominins dropped out of the trees and started to carry food and tools in their hands. These new species were poised to alter the planet unlike any other in the centuries to come.